The present invention relates to a press-molding device for molding lenses which are less in tilt of their respective optical axes and in shift or deviation of the optical axes.
In recent years, studies and researches have been conducted extensively, in which glass preform is press-molded to manufacture an optical lens directly.
A molding device for carrying out the above processing is known from, for example, Japanese Patent Application Laid-Open No. 60-118640. The molding device comprises a sleeve having a peripheral wall formed therein with a takeout opening, and a pair of cope and drag or upper and lower mold portions slidably accommodated in the sleeve. A ring-like mold portion, to which a glass preform is mounted, is inserted into the sleeve through the takeout opening. The glass preform is press-molded within the ring-like mold portion by the cope and drag.
The above-mentioned device has the following problem. That is, a clearance of the order of 4 to 10 micrometers is required between the cope and drag and the sleeve in order to enable the cope and drag to slide within the sleeve. Because of the clearance, the cope and drag tilt with respect to the sleeve at the pressing. In case of an aspheric lens for which severity is requested in connection with a tilt of its optical axis, the cope and drag and the sleeve must be lengthened considerably to reduce the tilt. This results in a increase of the overall dimension of the device. Further, since the arrangement of the device is such that the cope and drag are fitted from above and below into the ring-like mold portion inserted into the sleeve, it is essential that a certain measure of clearance is provided between the cope and drag and the ring-like mold portion. Thus, it is difficult from the viewpoint of the fundamental construction to reduce the tilt of the optical axis of the lens and to bring the optical axis of the lens in conformity with a central axis thereof.
On the other hand, a lens having opposite aspheric faces has been developed as a lens of a pickup for a compact disc. The lens is extremely small such as 5 to 8 mm in outer diameter and, accordingly, high accuracy or precision is required for the lens faces, the eccentricity, and the central thickness. In case of a spherical lens, an eccentricity that is deviation of an optical axis of the lens with respect to a central axis thereof becomes a problem. In case of a lens having opposite aspheric faces having their respective central axes, however, a tilt between both the central axes and deviation or decentering between both the central axes become a problem. If an attempt is made to obtain a lens for the compact disc whose wave front aberration is within a range of 0.04.lambda. rms where .lambda. is the wavelength, it is necessary to restrain the tilt to a value within sixty (60) seconds, and the decentering to a value within 10 micrometers. In addition, it is required to restrain the thickness accuracy to a value within .+-.15 micrometers.
It is difficult for the conventional device, however, to obtain the above-mentioned lens. Thus, it has been desired to mold such lens by means of a device simple in construction.